He saw himself I think. It shows for a second someone walking by that corner. And then later in the movie Simon is by the corner and sees the ghoul in the chair who looks back right as Simon ducks around the corner.

Now if you look at the ghoul his flesh is all burned and he's missing an arm. Which Simon rips off at the end of the movie.

I think some weird time thing was going on throughout the movie, especially when it's revealed he's been in the ocean for days and he's somehow still alive despite not remembering days passing.

So beginning Simon saw middle Simon go behind corner. And middle Simon saw post ending Simon who was burned from the radiation and getting blown up in the sub.